Many electronic devices send and receive streams of data through data transmission cables according to a wide variety of communication standards. Like all electronic devices, such devices require electrical power in order to operate. In some situations, however, it may be desirable to place such a device, and extend a data transmission cable to it, in a location where an electrical power source is not readily available. It is advantageous in many such cases, therefore, to use the data transmission cable to supply the electrical power to the electronic device. In this manner, it is not necessary to install a separate power outlet near the electronic device, and greater flexibility is achieved in the selection of locations for such devices, such as internet phones, cameras and wireless access points.
Some example systems for transmitting both data and power over conventional data transmission cables include devices defined by the “Power over Ethernet” (PoE) standards. In general, the PoE standards define power sourcing equipment (PSEs) and powered devices (PDs), such that a PSE supplies electrical power and data through Ethernet cables to one or more PDs. The PSE is typically part of a switch, router, hub or other appropriate network communication device. The PD is typically part of an internet phone, a networked camera, a wireless access point or other appropriate type of networked device that communicates through the network communication device to other networked devices. The PoE standards are defined by the IEEE 802.3af PoE standard (ratified June, 2003) and the IEEE 802.3at PoE standard, sometimes called “POE+”, (ratified September, 2009). The TPS23754, TPS23754-1 and TPS23756 High Power/High Efficiency PoE Interface and DC/DC Controllers available from Texas Instruments Incorporated are examples of PoE PD interfaces and power controllers that may be incorporated into Ethernet-based devices to enable PoE functionality therein.
PoE devices are also compatible with the Ethernet standards that do not include power delivery over the Ethernet cables. Therefore, it is possible to connect a non-PoE Ethernet device (not shown) to a PSE via an Ethernet cable and transmit only data across the cable. In this case, it is undesirable to supply power through the cable to the non-PoE device, because the electrical power could damage the non-PoE device.
As a safeguard against damaging non-PoE Ethernet devices upon connecting them to a port of the PSE, the default mode for the PSE when no device is connected to an individual port thereof is to maintain the power turned off to that port, i.e. in an idle state. During this time, though, the PSE probes each of its idle ports every one to two seconds to determine whether a device has been connected to it. Thus, upon connecting an Ethernet device to one of the ports, the PSE detects the presence of the device through a handshaking procedure. The handshaking procedure enables the PSE to identify the Ethernet device as PoE-compliant before turning on power to the device and to maintain the power turned off when the Ethernet device is identified as not PoE-compliant. The PSE typically identifies a newly connected PoE PD within about one to two seconds of plugging the PD into an idle port.
While the PoE PD is connected to the port of the PSE and is generally operating normally, the fact that the PD draws current through the port enables the PSE to make an ongoing determination that power is to be maintained to the port. Therefore, when the PD is disconnected from the port of the PSE, the PSE detects the lack of current flow and, in response, turns off the power to that port. In this manner, the PSE is ready for another device to be connected, regardless of whether the new device is PoE-compliant or non-PoE-compliant.
In order to minimize power consumption and cost, it is desirable for the PoE PD to be able to enter a low power (e.g. sleep, hibernation or “green”) mode at various times. However, in order to make sure that the PSE does not turn off the power to the PD when the current drawn by the PD falls so low that the PSE might determine that the PD has been disconnected, the PoE standards call for the PD to comply with a Maintain Power Signature (MPS) requirement. The MPS is a minimal electrical signature (e.g. a minimum current drawn with a maximum impedance/resistance) continuously or periodically presented by the PD to the PSE after the PSE turns on the power for the PD to assure the PSE that the PD is still present. Thus, the MPS prevents the PSE from incorrectly determining that the PD has been disconnected and inadvertently turning off the power to the PD.
To maintain the minimum current requirement of the MPS, the PD generally has an MPS circuitry 100, as shown in an example in FIG. 1. The MPS circuitry 100 is often part of a larger component, such as a DC/DC converter, within the overall networked device. The MPS circuitry 100 generally includes an input supply voltage 102 (derived from the PSE voltage), fixed loads 104 and variable loads 106.
Powered by the input supply voltage 102 the fixed load 104 draws a steady current either all the time or only while the networked device is in its low power mode. The steady current draw of the fixed load 104 is sufficient to satisfy the MPS requirement, so the PSE does not incorrectly determine that the PD has been disconnected.
The variable load 106 represents other components that draw current in the PD, besides the fixed load 104, when the networked device is in its low power mode. For example, the variable load 106 may include an LED that is powered in order to provide a visual indication that the networked device is in the low power mode. Additionally, other circuitry may remain active, so a user or a separate electronic device (not shown) can cause the networked device to come out of the low power mode when needed.
The variable load 106 does not typically maintain a constant current draw. In fact, the current drawn by the components of the variable load 106 may vary widely and unpredictably, due to a variety of causes. Thus, it is not possible to design the fixed load 104 simply to make up whatever difference (between the PoE MPS requirements and the current drawn by the variable load 106) is needed to satisfy the PoE MPS requirements. Instead, the fixed load 104 must be able to account for the entire MPS, since the variable load 106 is insufficiently reliable or predictable to maintain a proper MPS. By designing the fixed load 104 to account for the entire MPS, however, the power usage by both the variable load 106 and the fixed load 104 will often put the PD well over the MPS minimum. As a result, there may be significant unnecessary waste of power by the combined operation of the variable load 106 and the fixed load 104. This waste of power renders the green or low power mode highly inefficient.